Solderable stainless steel

ABSTRACT

Means and method for rendering solderable a stainless steel body resistant to corrosive attack from common electrolytes. The method includes the steps of covering at least one surface of a stainless steel body with layers of metal including a first metal layer selected from the group consisting of nickel, cobalt and chromium which serves as a barrier layer to the diffusion of a subsequent metal layer or layers therethrough to the stainless steel body. The subsequent metal layer is resistant to corrosive attack from common electrolytes and is solderable. The subsequent metal layer consists of silver alloyed with a noble metal selected from the group consisting of gold, palladium, platinum, rhenium and osmium. The alloy layer may be formed by covering the barrier layer with a layer of silver and then a layer of gold or one of the other metals. The layered stainless steel body is heated to a temperature below the melting point temperature of the metal layer having the lowest melting point so that the layers, with the exception of the barrier layer, diffuse into one another, thereby forming the alloy layer overlaying the barrier layer which is resistant to corrosive attack from common electrolytes and is solderable. The stainless steel body is rendered solderable.

United States Patent 72] Inventors Edwin Russell Koons Whiteland;- Ralph G. Parker, Shelbyville; Jeffrey P. Rupley, Lawrence, all of Ind. [21] Appl. No. 871,344 [22] Filed Nov. 5, 1969 [45] Patented Jan. 11, 1972 [73] Assignee P. R. Mallory 8: Co., Inc.

Indianapolis, Ind. Original application Feb. 14, 1968, Ser. No. 705,372, now Patent No. 3,515,950, dated June 2, 1970. Divided and this application Nov. 5, 1969, Ser. No. 871,344

[54] SOLDERABLE STAINLESS STEEL 7 Claims, 5 Drawing Figs.

[52] US. Cl 29/196.6, 29/195, 204/37 [51] Int. Cl B32b 15/18 [50] Field of Search 29/1 96.6, 196

[56] References Cited UNITED STATES PATENTS 2,999,216 9/1961 Steiger et al. 29/l96.6 X

3,364,064 l/l968 Wijburg ABSTRACT: Means and method for rendering solderable a stainless steel body resistant to corrosive attack from common electrolytes. The method includes the steps of covering at least one surface of a stainless steel body with layers of metal including a first metal layer selected from the group consisting of nickel, cobalt and chromium which serves as a barrier layer to the diffusion of a subsequent metal layer or layers therethrough to the stainless steel body. The subsequent metal layer is resistant to corrosive attack from common electrolytes and is solderable. The subsequent metal layer consists ofsilver alloyed with a noble metal selected from the group consisting of gold, palladium, platinum, rhenium and osmium. The alloy layer may be formed by covering the barrier layer with a layer of silver and then a layer of gold or one of the other metals. The layered stainless steel body is heated to a temperature below the melting point temperature of the metal layer having the lowest melting point so that the layers, with the exception of the barrier layer, diffuse into one another, thereby forming the alloy layer overlaying the barrier layer which is resistant to corrosive attack from common electrolytes and is solderable. The stainless steel body is rendered solderable.

PAIENIEU JIIIII I I972 CLEANING STAINLESS STEEL BODY DIFFUSING THE SECOND AND THIRD METAL LAYERS INTO EACH OTHER THEREBY RENDERING THE 33 l STAINLESS STEEL BODY SOLDERABLE 3o 32 33 I Ir j S I. i

F165 f 5s 3 v 55 so Q 5o 62 42 (4 4 W 37 43 44 I I I All I L f ,m. 2 39 3 $17 55% III a I \59 x49 l ifffff 5 FIG 5 Ffl'fi INVENTORS EDWIN R. KOONS RALPH e. PARKER,JR,

ATTORNEY SOLDERABLE STAINLESS STEEL This is a division of application Ser. No. 705,372, filed Feb. 14, 1968, now Pat. No. 3,515,950, dated June 2, 1970.

The present invention relates to a means and method for rendering solderable a stainless steel body useful for, inter alia, the metal portion of a glass-to-metal seal in a wet electrolytic capacitor resistant to corrosive attack from common electrolytes.

Stainless steels include additions of nickel and chromium which improve the corrosion resistance of the major constituent iron. Due to the improved resistance of iron to corrosive attack from common electrolytes when about 1 1 percent by weight or more of free chromium and nickel and other alloying metals are added to the iron, it is thought that the use of such stainless steel as an end seal for capacitors using an electrolyte having a concentration of about 39 percent or more of sulfuric acid would be advantageous. Other electrolytes are hydrochloric acid, perchloric acid, phosphoric acid, acetic acid, formic acid, oxalic acid, popanoic acid, a combination of selenous acid-selenic acid-sulfuric acid and the like.

Stainless steels fall into various categories in accordance with the amount of chromium and nickel mixed with iron. The stainless steel body may also include minor additions of other metals such as columbium, tantalum, manganese, phosphorous, silicon, copper and the like to improve selected properties of the stainless steel such as corrosion resistance and the like. However, the mechanism which protects these alloys from environmental and chemical attack also prevents the effective cleaning of the surfaces of fluxes so that chemical preparation of the surface of stainless steel for wetting is difficult even under conditions where the stainless steel is not subjected to an extraordinary treatment such as that required for preparing a glass-to-metal seal generally used with an hermetically sealed wet electrolyte capacitor.

A glass-to-metal hermetic seal is usually formed by selecting a ring of suitable metal and filling the center of the metal ring with suitable glass frit. A metal wire extends through the glass frit and is used as a lead wire for a capacitor body. The metal ring, the glass frit and the lead wire are heated in situ at a sufficiently high temperature to melt the glass and render the viscosity of the glass such as to cause the molten glass to flow. In the case of compression glass-to-metal seals, the temperature to which the components of the seal are heated is usually about 930 C. or above in order to render the glass molten. The glass and metal members of both compressive and noncompressive seals are so constrained that the glass flows only radially to fill the entire space between the lead wire projecting through the glass and the metal ring. A glass-to-metal seal suitable for use as an hermetic seal is provided upon cooling of the molten glass.

It was found that when stainless steel resistant to corrosive attack from common electrolytes such as sulfuric acid is subjected to the high temperature necessary to render the glass frit molten, a thermal oxide film or coating is formed thereon which renders joining of the stainless steel to a solderable body more difficult than the usual difficulty encountered with the soldering ofstainless steel to a solderable body. The means and method of the present invention permit the use of stainless steel in glass-to-metal hermetic seals yet the seal exhibits excellent solderability characteristics.

Therefore, it is an object of the present invention to provide a solderable stainless steel body including a body of stainless steel coated with materials so as to substantially prevent thermal oxidation of the stainless steel body during the formation of a glass-to-hermetic seal thereby improving the solderability characteristics of the resultant glass-to-stainless steel seal.

With the aforementioned objects enumerated, other objects will be apparent to those persons possessing ordinary skill in the art. Other objects will appear in the following description, appended claims and appended drawings. The invention relates to the novel construction, combination, arrangement and cooperation of elements as hereinafter described and more particularly as defined in the appended claims.

The appended drawings illustrate an embodiment of the present invention constructed to function in the most advantageous modes devised for the practical application of the basic principles involved in the hereinafter described invention.

In the drawings:

FIG. 1 illustrates the several steps of a process for rendering solderable stainless steel used in a glass-to-metal seal.

FIG. 2 is a diagrammatic side view of two parts, one of which is stainless steel having the surfaces thereof prepared in accordance with the method of the present invention and joined to a solderable material;

FIG. 3 shows an embodiment of the present invention wherein two stainless steel parts prepared in accordance with the method of the present invention and joined to each other by diffusion;

FIG. 4 is a sectional view of a capacitor using a liquid electrolyte and having a glass-to-stainless steel hermetic seal closing the open end of the housing for the capacitor; and

FIG. 5 is a sectional view of a capacitor including a liquid electrolyte and an embodiment of the glass-to-stainless steel hermetic seal illustrated in FIG. 4.

Generally speaking the present invention relates to corrosion resistant stainless steel part rendered solderable by covering the part with at least two metal layers. The initial layer of metal substantially prevents the metal layer or other metal layers from diffusing therethrough to the stainless steel. The other metal layer or layers are solderable and resistant to chemical attack from common types of electrolytes.

It was found that during the fabrication of the glass-to-metal seal, a stainless steel resistant to corrosion attack from a sulfuric acid electrolyte formed a thermal oxide which rendered the stainless steel unsolderable using conventional stainless steel soldering techniques. The temperature used to form the glass-to-metal seal is about 930 to 950. The corrosive resistant stainless steel consists essentially of about 2 percent by weight manganese, about 1 percent by weight silicon, 19-21 percent by weight chromium, about 3038 percent by weight nickel, about 2-3 percent by weight molybdenum, about 3-4 percent by weight copper with minor additions of columbium, tantalum, phosphorus and sulfur, and the remainder iron. The stainless steel fabricated from the above listed constituents is resistant to chemical attack from hot sulfuric acid electrolytes having a concentration ofup to 40 percent.

Referring now to the figures of the drawings, FIG. 1 illustrates the several steps of the method involved in the present invention. A stainless steel body resistant to corrosive attack from sulfuric acid electrolyte is thoroughly cleaned by any suitable electro cleaning method of the like. The stainless steel body may have a ringlike configuration if the stainless steel is to be used as the metal portion of a glass-to-metal seal in 2 hermetically sealed capacitor. The stainless steel body is electroplated with a barrier layer having a sufficient metal thickness to prevent diffusion therethrough of a subsequent metal layer or layers. The barrier layer is selected from the group consisting of nickel, cobalt and chromium. Of the several metals, nickel is most preferred. If nickel is used, the thickness of the nickel layer is about 0.0002 to 0.0004 inches thick. A nickel layer having a thickness of about 0.0003 inches thick is preferred. Electroplating a metallic barrier layer of nickel less than 0.0002 inches thick seriously subtracts from the ability of the nickel barrier layer to prevent a subsequent metallic layer or layers from diffusing therethrough to the stainless steel body. The upper limit of the thickness of the nickel barrier layer may be exceeded, however, no tangible benefit appears to result therefrom.

The barrier layer metal is electroplated with a silver layer having a thickness of about 0.0008 to 0.002 inches thick. A layer of silver having a thickness of about 0.001 inches thick is preferred. Electroplating a silver layer over the barrier layer metal having a thickness of less than 0.0008 inch undermines the ability of the silver layer to prevent the subsequent metal layer or layers from diffusing therethrough. Exceeding a silver layer thickness of about 0.002 inch does not appear to result in any tangible benefits nor does exceeding the upper limit thickness result in a condition detrimental to the system. Copper may be substituted for silver in situations where the stainless steel is used in an environment where sulfuric acid is not present.

The silver layer is electroplated with a noble metal selected from the group consisting of gold, palladium, platinum, rhenium and osmium. Of the several metals used gold is preferred, and 24 kt. gold is most preferred. The thickness of the gold layer is about 0.00005 to 0.0002 inches thick. A gold layer having a thickness of about 0.0001 ispreferred. A 24 kt. gold layer having thickness of less than about 0.00005 inches thick results in detrimental oxidization of the underlying silver layer during treating of the plated stainless steel body to cause the silver and gold to diffuse into one another. If the silver oxidized to an appreciable extent, the solderability of the stainless steel is seriously undermined. Exceeding a gold thickness of about 0.0002 inch does not appear to result in any tangible benefit nor does the increased thickness of the gold layer result in a condition detrimental to the system.

The plated stainless steel body is heated to a temperature of about 800950 C. for approximately to 30 minutes and allowed to cool slowly. A temperature of about 930950 C. for about 15 to minutes is preferred for the diffusion step and glass melting step may be conveniently performed simultaneously. During the heating of the plated stainless steel part to a temperature of about 800-950 C. there is affected a diffusion of the noble metal layer or film into the silver layer or film underneath the noble metal film so as to provide a layer of silver-noble metal alloy. Diffusion of the silver layer and the gold layer into each other appears to result in a silver-gold alloy having a thickness greater than the sum total of the thicknesses of the individual silver layer and the gold layer. For example, a silver layer having a thickness of about 0.00108 inches and a gold layer having a thickness of about 0.00010 inches form a swollen silver-gold alloy having a thickness of about 0.00132 inches. In another case, a silver layer having a thickness of about 0.00111 inches and a gold layer having a thickness of about 0.00010 inches form a swollen silver-gold alloy having a thickness ofabout 0.0123 inches. In the first example, the alloy has an increased thickness of about 16 percent greater than the sum total of the thickness of the silver layer and the gold layer. In the second example, the alloy has an increased thickness of about 1 percent greater than the sum total of the thicknesses of the silver layer and the gold layer. It is thought that the interaction of the metals during the diffusion step causes random imperfections in the alloy. However, the imperfections do not extend through the alloy layer.

The preferred alloy of silver-gold contains about 80-90 percent by weight silver, the remainder essentially gold with minor amounts of impurities. The most preferred composition of the silver-gold alloy is about 88 percent by weight silver, the remainder essentially gold with minor amounts of impurities. It should be recognized that the percent by weight of the gold content of the alloy could be increases significantly above 20 percent by weight, but the fact that not additional benefits are realized does not appear to justify the increases cost associated therewith. The apparent limitation of the gold content of the alloy is that a minimum thickness of silver is required to be electroplated over the barrier layer metal to substantially prevent diffusion of the gold to the barrier layer metal.

The silver and noble metal layers prevent thermal oxidization of the stainless steel during subjection of the stainless steel to the elevated temperatures required for forming the glass-to-metal seal. The silver-noble metal alloy is resistant to chemical corrosion from the sulfuric acid electrolyte, does not form a thermal oxide when subjected to the elevated temperatures required for the formation of the glass-to-metal seal and is easily solderable to any material solderable under normal conditions such as zinc, iron. cadmium, indium, nickel, tin,

lead, animony, bismuth, copper, steel, silver, palladium, gold and platinum or a metal plated with said metals. It is thought, however, that for the purposes of the intended use of the present invention only those metals not subject to corrosive attack from the sulfuric acid electrolyte would be acceptable. Materials such as silver, palladium, platinum and gold or metals plated with said materials such as for example silver plated stainless steel, silver clad stainless steel and the like would be acceptable.

It is though that any solder which is resistant to corrosion attack from the sulfuric acid electrolyte would be satisfactory. Since the sulfuric acid electrolyte would be boils at a temperature of about 105 C., the lower the melting point temperature of the solder, the more advantageous the solder. Of the several available solders, silver base solders are preferred because of their resistance to corrosive attack from the sulfuric acid electrolyte and because of their low melting point temperature. of the silver base solders, silver-lead solder is most preferred. The preferred composition of the silver-lead solder is approximately percent by weight or more lead, the remainder silver. Of the possible lead-silver alloys, however, a solder having approximately 97.5 percent by weight load, the remainder silver is most preferred.

By way of example, a solderable glass-to-stainless steel and seal for an electrolytic capacitor using an electrolyte having about a 40 concentration sulfuric acid may be prepared by anodically cleaning a stainless steel ring of suitable dimensions in a suitable cleaning device. The stainless steel ring is immersed in a bath containing nickel and a Woods Nickel Strike is electroplated thereon having a thickness of about 0.00005 inch. The temperature of the bath is room temperature and the current density during electroplating is about 10 amperes per square foot. The electroplated stainless steel ring is rinsed in water. The electroplated stainless steel ring is immersed in a bath containing nickel sulfamate and having a temperature of about l25-l35 F. Nickel is electroplated onto the stainless steel using a current density of about 4 amperes per square foot. A nickel plate thickness of about 0.0003 inch is realized. The stainless steel ring or part is rinsed in water. A silver strike is electroplated over the nickel plate by immersing the part in a bath at room temperature and electroplating using a current density of about 5 amperes per square foot. The thickness of the silver strike is about 0.00005 inch. The part is rinsed in water. The part is immersed in a bath of mullosil silver at about room temperature. The part is electroplated with silver by using a current density of about 2.5 amperes per square foot. A silver thickness of about 0.001 inch is electroplated on the part. The part is rinsed in water. The part is placed in a bath containing gold at room temperature. Electroplating of gold is carried out using a current density of about 1 ampere per square foot. About 0.0001 inches of 24 kt. gold is plated on the part. The part is removed from the bath, dried and the outer portion of the ring is filled with a suitable glass frit. The part and the glass frit are heated to about 940 C. 10 C. for 15 to 20 minutes to cause the silver and gold layers to diffuse into one another thereby forming a silver-gold alloy which is resistant to corrosive attack from sulfuric acid electrolyte, which prevents the stainless steel from oxidizing during diffusion step, and which is solderable under normal conditions. The heating of the part and the glass frit for the specified time at the specified temperature cause the glass frit to melt and upon cooling, to form a glass-to-stainless steel seal.

Any one of several suitable glass frits may be used to provide the glass-to-metal seal. One of the limitations placed on the glass is that its melting point be less than the lowest melting point temperature of the metals electroplated over the stainless steel. It is thought that the maximum upper limit on the working point temperature of the glass is about 960 C. since the melting point temperature of silver is about 961 C. A minimum working point temperature of the glass is about 800 C. since it is thought that at last this temperature is required to cause the second and third layers to diffuse into one another Within practical lengths of time. It is recognized that the silver layer and the noble metal layer will diffuse into one another without heating if allowed to stand a sufficient length of time. If the diffusion step and the glass melting step are not performed simultaneously then the lower limit on the melting point temperature of the glass ceases to be of importance, A suitable glass for use when the diffusion and glass melting steps are performed simultaneously contains the following, by weight constituents: about 2832 percent silicon, about 20-23 percent sodium, about 4-5 percent by weight potassium, about -12 percent barium the remainder oxygen with traces of lead, chromium lithium, copper and tin.

It is thought that the diffusion step may be eliminated by electroplating silver and gold simultaneously from the potassium cyanide solutlon to thereby form a silver-gold alloy over the barrier layer metal. It is recognized that the constituents of solution will vary in accordance with the percent by weight of the constituents of the silver-gold alloy.

FIG. 2 shows a stainless steel part 10 which may be a component part of a glass-to-metal seal electroplated with a layer of nickel 11, and a layer of an alloy of silver-gold 12 wherein the silver and gold are diffused into one another. The silvergold alloy renders the stainless steel solderable to any suitable material solderable under normal conditions such as a silverclad stainless steel piece 13 by the use of any suitable solder 14 such as lead-silver solder. The joint between the stainless steel body and the solderable metal part is mechanically strong and resistant to chemical attack from a sulfuric acid electrolyte. The stainless steel piece may be crimped and soldered in the manner shown in FIG. 2 thereby providing a mechanically strong seal construction.

FIG. 3 shows another embodiment of the present invention wherein flanged stainless steel part 30 and stainless steel part 31, which are resistant to chemical attack by a sulfuric acid electrolyte, are each electroplated with a layer of nickel 32 and 32, a layer of silver and a layer ofgold in accordance with the method taught hereinbefore. The plated stainless steel parts are placed in abutting relationship and then heated to a temperature of about 800-950 C. for approximately 30 minutes and allowed to cool slowly, During the heating of the pieces or parts to the temperature of about 800-950 C. there is effected a rapid diffusion of the gold and the silver layers carried by each of the stainless steel parts into one another to form gold-silver alloy 33 and 33. Upon cooling of the respective parts, there is effected by such diffusion of the gold and silver layers or films into one another, a resultant gold-silver alloy which provides strong mechanical joint which is resistant to chemical attack by the sulfuric acid electrolyte. It should be noted that the use of solder to effect the joining ofthe stainless steel parts is eliminated.

FIG. 4 represents an electrical component such as a capacitor 39 including a glass-to-stainless steel assembly 40 which closes the open end of cathode can 49 thereby providing an hermetically sealed wet electrolytic capacitor. The seal assembly 40 includes film forming metal wire 41 having an oxide layer 47 formed thereon which is sealed to a glass ring 43 by heating a glass frit to a temperature of about 930-950 C. to cause the glass to become molten and form a compressive seal with the metal wire projecting therethrough upon cooling. THe glass ring 43 is in turn compressively sealed to the outer metal ring 44 of stainless steel. The stainless steel ring is resistant to corrosive attack from a sulfuric acid electrolyte 45. The electrolyte 45 is a solution which contains about 39 to 40 percent sulfuric acid. Prior to the assembling of the capacitor an anode riser 46 of any suitable film forming metal having an oxide layer formed thereon projects from an anode 48 of filmforming metal and is welded to the metal wire 41 at a point below the seal as shown at 47. The metal wire, the anode riser and the anode are fabricated from the same film-forming metal, that is, tantalum or the like. The anode 48 may be fabricated from film-forming metal foil rather than the pressed and sintered powder as shown in FIG. 4. Any resulting defects in the oxide film resulting from the weld operation are healed by the sulfuric acid electrolyte 45. The seal assembly 10 is then positioned in the noncorrosive cathode can 49 and soldered or brazed in place with a silver base solder 37 as described hereinbefore. A conductive lead wire (not shown) is welded to the lead wire 41 to complete the capacitor. The capacitor may include an anode spacer 38 for seating the anode. The spacer is fabricated from any suitable resilient material which is not chemically attacked by the electrolyte.

FIG. 5 shows yet another embodiment of the present invention using the solderable glass-to-stainless steel seal assembly 50. The capacitor 51 comprises a noncorrosive metal cathode can 52 having an anode spacer 53 fabricated from any suitable resilient material which is not chemically attacked by the electrolyte. The spacer is positioned in the closed end of the cathode can for seating the capacitor anode 54 fabricated from a suitable film-forming metal. The seal assembly 50 includes a hollow film-forming metal tube 55 having an oxide film 56 formed on the inner periphery thereof. A glass ring 61 circumscribes the tube and is compressively sealed to the outer periphery of the tube and to a noncorrosive metal ring 60 of stainless steel. The metals used in the can and the ring must be compatible with the entire system and capable of being welded together.

An anode riser 57 of a suitable film-forming metal extends from a film-forming metal anode S4 and projects through a portion of the aperture of the tube 55. The tube, the anode riser and the anode are fabricated from the same film-forming metal, that is, tantalum or the like. The anode 54 may be fabricated from a film-forming metal foil rather than the pressed and sintered powder as illustrated in FIG. 5. It should be noted that the riser has an oxide film 56' formed on the periphery thereof. The tube is pinched inwardly by any suitable means such as by crimping means and is welded at the crimp as shown at 58 in FIG. 5. The riser is not integral with and positioned by seal assembly 50. The seal assembly and the anode are then positioned within the cathode can containing a liquid electrolyte such as sulfuric acid. The anode spacer 53 is optional, as anode 12 is securely positioned by the crimp and the weld joint at the crimp. The seal assembly is soldered to the can using any suitable corrosive resistant solder such as lead-silver solder 62. The can may be fabricated from any noncorrosive material such as stainless steel, silver, silver plated stainless steel, silver clad stainless steel and the like.

While the invention is illustrated and described in an embodiment it will be understood that modifications and variations may be effected without departing from the scope of be novel concepts of this invention and as set forth in the appended claims.

Having thus described our invention, we claim:

1. A solderable part comprising a stainless steel part, a first layer which is a barrier to diffusion of materials therethrough to said stainless steel, and a second layer of a solderable material over said first layer thereby rendering said stainless steel part solderable.

2. A solderable part as claimed in claim 1, wherein said first layer is a metal selected from the group consisting of nickel, chromium and cobalt and said second layer is an alloy wherein one of the constituents thereof is selected from the group consisting of silver and copper and the other constituents of said alloy is selected from the noble metal group consisting ofgold, palladium, platinum, rhenium and osmium.

3. A solderable metal part as claimed in claim 2, wherein said first layer is about 0.0002 to 0.0004 inches thick and said alloy layer consisting essentially of said noble metal having an original thickness of about 0.00005 to 0.0002 inches and said first constituent having an original thickness of about 0.00008 to 0.002 inches.

4. A solderable metal part as claimed in claim 3, wherein the thickness of said alloy layer is about 1-16 percent greater than the total thickness of the layered constituents.

5. A solderable metal part as claimed in claim 2, wherein said alloy constituents are silver and gold.

6. A solderable metal part as claimed in claim 5, wherein said silver-gold alloy consists essentially of about 80-98 percent by weight silver, the remainder essentially gold.

7. A solderable metal part as claimed in claim 6, wherein said silver content of said alloy is about 88 percent by weight, the remainder essentially said gold. 

2. A solderable part as claimed in claim 1, wherein said first layer is a metal selected from the group consisting of nickel, chromium and cobalt and said second layer is an alloy wherein one of the constituents thereof is selected from the group consisting of silver and copper and the other constituents of said alloy is selected from the noble metal group consisting of gold, palladium, platinum, rhenium and osmium.
 3. A solderable metal part as claimed in claim 2, wherein said first layer is about 0.0002 to 0.0004 inches thick and said alloy layer consisting essentially of said noble metal having an original thIckness of about 0.00005 to 0.0002 inches and said first constituent having an original thickness of about 0.00008 to 0.002 inches.
 4. A solderable metal part as claimed in claim 3, wherein the thickness of said alloy layer is about 1-16 percent greater than the total thickness of the layered constituents.
 5. A solderable metal part as claimed in claim 2, wherein said alloy constituents are silver and gold.
 6. A solderable metal part as claimed in claim 5, wherein said silver-gold alloy consists essentially of about 80-98 percent by weight silver, the remainder essentially gold.
 7. A solderable metal part as claimed in claim 6, wherein said silver content of said alloy is about 88 percent by weight, the remainder essentially said gold. 